PoC: Add Variant type support in Iceberg

[aihuaxu]

This is to prototype the Variant type support in Iceberg. Spark is able to read and write Variant data to Iceberg table. Some of the changes e.g., Spark 3.5 should be new Spark 4.0, but this is to show the changes.

This PR includes:

• Add Variant data type in Iceberg API.
• Iceberg Parquet module to map Variant type to a struct type with Value and Metadata fields.
• Iceberg Spark module to support read and write Iceberg Variant.

I will try to break the changes into multiple PRs - API, Parquet and Spark changes.
Fix: #11178

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that’s incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the [email protected] list. Thank you for your contributions.

[github-actions]

This pull request has been closed due to lack of activity. This is not a judgement on the merit of the PR in any way. It is just a way of keeping the PR queue manageable. If you think that is incorrect, or the pull request requires review, you can revive the PR at any time.